1. Technical Field
This disclosure relates to various load-bearing medical implants with at least one electronic component that is sealed within a load-bearing structure of the implant to provide an impermeable barrier to protect the electronic component from body fluids. Various methods are disclosed for hermetically sealing the electronic component within a metallic load-bearing implant structure by welding a weld plate over the cavity that accommodates the electronic component without causing thermal damage to the encapsulant or electronic component. Various techniques are also disclosed for encapsulating an electronic component within a cavity of a load-bearing implant, such as an IM nail that includes one or more electronic sensors for landmark identification. The encapsulation and welding techniques disclosed herein address the problems associated with load-bearing implants having shallow cavities for sensor or other components and shallow channels for wiring, wherein the sensor and encapsulant can be damaged by welding a cover plate in close proximity to the enscapsulant and sensor.
2. Description of the Related Art
While most orthopedic implant developers are focused on improving current technologies, a handful are directed to developing “smart” or “intelligent” orthopedic implants equipped with implantable electronic components. Such electronically-equipped orthopedic implants provide real-time feedback to researchers, physicians or patients regarding how the implants are performing once they are placed inside a bone or joint. For example, orthopedic implants with electronic components can be used to detect poor bone in-growth, educate patients about safe post-operative activities, and improve surgical techniques.
The implantable electronic circuits and components must be small to minimize the size of the implant and designed to last in a physiological environment for an extended period of time. A reliable hermetic barrier must be used to preventingress of body fluids to the implantable electronic components and to assure long term biocompatibility. Generally used methods for protecting electronic circuits from the bodily fluids or other damaging environments include both hermetic sealing and polymer encapsulation.
Encapsulants, such as silicone elastomers, polyurethanes, silicone-urethane copolymer, polytetrafluoroethylene and epoxies have been used with implantable neuromuscular stimulators which rely on relatively simple circuits. However, polymers do not provide an impermeable barrier and therefore cannot be used for encapsulation of high density, high impedance electronic circuits. The moisture ingress will ultimately reach the electronic component resulting in electric shorting and degradation of leakage-sensitive circuitry.
For radio frequency powered electronic components disposed within a medical implant, a combination of hermetic packaging and polymer encapsulation are used. Hermetic packaging, using metals, ceramics or glasses, provides the implant electronic circuitry with a long term protection from the ingress of body fluids. The primary role of the encapsulant is to stabilize the electronic components by acting as stress-relieving shock and vibration absorbers and providing electrical insulation. Electrical signals, such as power and stimulation, enter and exit the implant through hermetic through-holes, which are hermetically welded into the implant walls. The through-hole assembly utilizes a ceramic or glass insulator to allow one or more wires to exit the implant.
In certain situations, electrical through-holes are not practical due to limited design space (e.g., <1 mm diameter) available for the parts in combination with the risk of fatigue failure of the connection due to cyclic loading of the implant. As a result, the role of the encapsulant as a secondary barrier to body fluid ingress becomes more important. Such devices include intramedullary (IM) nails, plates, rods and pedicle screws for orthopedic trauma application. In order to increase the body fluid barrier characteristics of the flexible impermeable encapsulant, the cavities that hold the electronic components need to be completely filled. This is difficult to achieve if the weld plate components have to be welded in close proximity with the encapsulant and the cavities are too long and narrow to allow adequate backfilling after hermetic sealing.
Currently available medical grade silicone encapsulants are only suitable for short-term (e.g., <30 days) implantable applications, referred to as “restricted grade.” However, some materials, such as MED3-4213 and ELAST-EON™ developed by NuSil Silicone Technology (www.nusil.com) and AorTech (www.AorTech.com) respectively are unrestricted grades of silicone for long term implantation. Given that the onset temperature of thermal degradation for these types of materials is approximately 230° C., standard welding techniques, which generate local temperatures in the 400° C.-600° C. range, are not appropriate without the risk of degradation of either mechanical or optical properties the silicone. When exposed to high temperature conditions, the silicone will degrade leading to unpredictable performance.
Scanning electron microscope (SEM) micrographs of cured MED3-4213 encapsulated in an implant before and after conventional welding techniques are shown in FIGS. 1A-1E. It is evident from FIGS. 1A-1E that performance degradation resulting from increases in optical absorption are noticeable in the form of a hazy or milky appearance that is apparent from a comparison of FIG. 1A, which shows a layer of undamaged silicone, and FIGS. 1B-1E. Furthermore, mechanical degradation takes the form of voids 22 (FIG. 1B), pitting 23 (FIG. 1C), degraded portions 24 of the polymer near the welding zones 25 (FIGS. 1D-1E), hardening/denaturizing, out gassing of volatiles, brittle structures, crazing, cracking, shrinking, melting, or delamination. Accordingly, all of these problems compromise biocompatibility and mechanical performance of the implant.
There are no existing medical grade elastomers that can meet the high temperatures (400° C.-600° C.) needed for conventional welding which is used to provide a hermetic seal in the form of a weld plate over the cavity accommodating electronic component. As a result, a more cost-effective solution would be to optimize the existing methods of hermetic sealing. Consequently, there is a need for improved methods of packaging electronic components within an encapsulant that overcomes the thermal degradation issue caused by conventional welding techniques used to provide a hermetic seal. This need applies to medical implants and other unrelated applications.